Inferences from pulmonary O2 uptake with respect to intramuscular [phosphocreatine] kinetics during moderate exercise in humans

Abstract

1. In the non-steady state of moderate intensity exercise, pulmonary O2 uptake (Vp,O2) is temporally dissociated from muscle O2 consumption (Vm,O2) due to the influence of the intervening venous blood volume and the contribution of body O2 stores to ATP synthesis. A monoexponential model of Vp,O2 without a delay term, therefore, implies an obligatory slowing of Vp,O2 kinetics in comparison to Vm, O2. 2. During moderate exercise, an association of Vm,O2 and [phosphocreatine] ([PCr]) kinetics is a necessary consequence of the control of muscular oxidative phosphorylation mediated by some function of [PCr]. It has also been suggested that the kinetics of Vp,O2 will be expressed with a time constant within 10 % of that of Vm,O2. 3. Vp,O2 and intramuscular [PCr] kinetics were investigated simultaneously during moderate exercise of a large muscle mass in a whole-body NMR spectrometer. Six healthy males performed prone constant-load quadriceps exercise. A transmit-receive coil under the right quadriceps allowed determination of intramuscular [PCr]; Vp,O2 was measured breath-by-breath, in concert with [PCr], using a turbine and a mass spectrometer system. 4. Intramuscular [PCr] decreased monoexponentially with no delay in response to exercise. The mean of the time constants (tauPCr) was 35 s (range, 20-64 s) for the six subjects. 5. Two temporal phases were evident in the Vp, O2 response. When the entire Vp,O2 response was modelled to be exponential with no delay, its time constant (tau'Vp,O2) was longer in all subjects (group mean = 62 s; range, 52-92 s) than that of [PCr], reflecting the energy contribution of the O2 stores. 6. Restricting the Vp,O2 model fit to phase II resulted in matching kinetics for Vp,O2 (group mean tauVp,O2 = 36 s; range, 20-68 s) and [PCr], for all subjects. 7. We conclude that during moderate intensity exercise the phase II tauVp,O2 provides a good estimate of tauPCr and by implication that of Vm,O2 (tauVm,O2).

Figure 1. Schematic representation of exercise in the NMR system

The ³¹P surface coil is placed under the quadriceps of the dominant leg and the subject breathes through the mouthpiece containing the tip of the extended sampling line and the non-ferrous turbine volume sensor. The output from the turbine passes through a low-pass filter, which is earthed to the Faraday cage, and into the volume measuring module (VMM).

Figure 2. The ‘stack plot’ of phosphorus metabolites in the exercising quadriceps from one representative subject (subject 2)

The reference peak, PCr, is set to 0 p.p.m. and each spectrum represents 15 s. The figure has been produced with the x-axis inverted such that the P_i peak may be visible in front of the large PCr peak.

Figure 3. The responses of one representative subject (subject 1) to a constant-load change in work rate of a moderate intensity of V̇_p,O2 and [PCr]

A and B show the same V̇_p,O2 data modelled with different strategies; C shows the simultaneously determined [PCr]. The data in A are fitted with a monoexponential function beginning at the onset of exercise (model 1). The data in B are fitted with a monoexponential function that begins after the φI-φII transition and does not include the φI response in the fitting procedure (model 2). The data in C are fitted with a monoexponential function constrained to begin at the onset of exercise.

Figure 4. The responses of V̇_p,O2 and [PCr] to a step change in work rate of moderate intensity for one subject (subject 1)

The responses are superimposed to show the dissimilarity, in the case of A, and similarity, in the case of B, of the kinetic responses of the two variables. A shows the V̇_p,O2 (•) fitted by a model 1 exponential. B shows the V̇_p,O2 (•) phase shifted and fitted by a model 2 exponential. The model 2 fit of V̇_p,O2 closely represents the kinetic response of [PCr] (^) to the exercise (B).

Figure 5. The kinetic responses of V̇_p,O2 and [PCr] to a constant-load change in work rate in six subjects

A shows V̇_p,O2 with the φII response fitted by a monoexponential. B shows the simultaneously determined [PCr] response. C shows the identity of φII V̇_p,O2 (•) and [PCr] (^) kinetics determined simultaneously during quadriceps exercise. The [PCr] scale has been inverted to facilitate kinetic comparisons. Graphs from top to bottom show data from subjects 1 to 6, respectively.

Figure 6. Comparisons of the time constants estimated by model 1 (τ‘) and model 2 (τ) with the simultaneously determined τ_PCr

A, the time constants (τ) of φII V̇_p,O2 (model 2) and [PCr] lie within ≈10 % of identity. The error bars give the 95 % confidence intervals of the prediction of τ for each subject. B, the mean response time of V̇_p,O2, τ′ (model 1), systematically overestimates the time constant for the fall in muscle [PCr].